I understand what scrub radius is and how it affects the handling of the car. What I dont understand is how it can be practically measured or calculated. In this thread, the 1g guys determined that the proper offset for 17x9 wheels is +27 to maintain a proper scrub radius. I do not how this offset was determined, if this carries over to a 2g, or how this changes if a different wheel width is chosen. Did they change the scrub radius from stock intentionally for a better handling car on a track?

Also, given a situation where one can have either wider tires (within reason) with a less than ideal scrub radius, or skinnier tires (within reason) with an ideal scrub radius, which is preferable in road race and autocross?

Scrub radius, to my knowledge, cannot be calculated to an exact number. It is only referred to as positive or negative. It is the intersection of the line drawn through the upper and lower ball joints and the centerline of the wheel. If the intersection of this line is above the surface of the road, it is said to be negative scrub. If it is below the road, it is positive.

So, by putting wider wheels on your car, and as a result (this is where offset matters), moving the centerline of the wheels outward, a likely result is positive scrub, since the point of intersection is moved further outward, and as a result, lower to/in the ground. You can counter act the wider wheels by moving the upper ball joint inward.

On the point of tire width, there will be no difference in scrub if using the same wheels, since the centerline will not change. Taller tires, on the other hand, can affect scrub.

If scrub radius can not be measured precisely, then there must be some way to determine the proper offset for a new wheel relative to the stock wheel offset and width. Or for a new suspension configuration relative to stock (since ride height, camber, etc. will affect it). Or both at the same time.

From my research, all I've seen is people saying to keep scrub radius in mind when choosing an offset for an aftermarket wheel. And nothing more is said.

I couldn't really tell you a good way to measure it, other than eyeballing, or taking a picture of the suspension with it fully loaded, and then you can draw with a straight edge. Keep in mind, also, that scrub will change (although minutely), with any change in ride height of camber. If both of these are stock, the best way to maintain your scrub is to make sure that your new wheels' centerline is in the same location in relation to the suspension as the stock ones. I can't remember where I saw it, but I know a member on here had redrilled his upper control arms so that they were actually shorter, giving him more static camber for road racing. This is also a great way to counter act the negative effects of wider wheels that stick out further.

I can't imagine eyeballing it will work. We're talking the difference of millimeters here.

My car will be slightly lower than stock, and will have more negative camber with the SPC upper control arms.

Now here's what I don't understand: let's consider the stock wheel (17x6.5 +46), and an aftermarket wheel (17x9.5 +??). Wouldn't I have to use a +46 offset to keep the center of the wheel in the same position as stock? (Obviously, that's not possible with a 17x9.5 wheel, but let's say it is.) Since offset is defined as the distance from its hub mounting surface to the centerline of the wheel, the only way to keep an identical scrub radius (if everything else is a constant) is to keep the same offset. Or am I way off here?

So I'm still confused how the 1g guys came up with an offset of +27 on a 17x9 wheel when stock is +46.

Well, hrm. I feel like it would take a lot of specialty measuring devices to acurately dial in something like that. I have no idea how they decided the appropriate offset in the other thread. I suppose with precise measurements of the control arms, wheel dimensions, and ride height, you could calculate optimum wheel offset, but it would certainly take a lot of work to do so.

So I'm still confused how the 1g guys came up with an offset of +27 on a 17x9 wheel when stock is +46.

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I've been thinking about this all day. I can't figure it out either.

Maybe they weren't really concerned with scrub radius foremost?
They definitely were trying to go wider without any fear of not being able to fit.

If you have a 225mm wide tire with an offset of +46, the mating surface of the wheel is 158.5mm from the inside edge of the tire.

For a 255mm wide tire to keep that 158.5mm of inside clearance, you'd put the wheel offset at +31mm.

For a 275mm wide tire to keep 158.5mm inside, you'd put the wheel offset at +21mm.

So those theoretical offset numbers are semi-close to putting the 1G wheels in the middle of the range. Perhaps clearance was actually the primary concern? Maybe just finding a lightweight wheel in the right lug pattern, hub centric to 67.1, with good brake clearance was hard enough to justify a compromise in the scrub radius? Because the scrub radius change is obviously going the wrong way with a change to +27 from +46.

Anyone know how to get in touch with Mitch? I'd be very interested to know how he chose those.

Maybe they weren't really concerned with scrub radius foremost?
They definitely were trying to go wider without any fear of not being able to fit.

If you have a 225mm wide tire with an offset of +46, the mating surface of the wheel is 158.5mm from the inside edge of the tire.

For a 255mm wide tire to keep that 158.5mm of inside clearance, you'd put the wheel offset at +31mm.

For a 275mm wide tire to keep 158.5mm inside, you'd put the wheel offset at +21mm.

So those theoretical offset numbers are semi-close to putting the 1G wheels in the middle of the range. Perhaps clearance was actually the primary concern? Maybe just finding a lightweight wheel in the right lug pattern, hub centric to 67.1, with good brake clearance was hard enough to justify a compromise in the scrub radius? Because the scrub radius change is obviously going the wrong way with a change to +27 from +46.

Anyone know how to get in touch with Mitch? I'd be very interested to know how he chose those.

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The TD wheels were chosen and made to that spec. As far as I can remember, Mitch was working with TD directly to determine the correct specs to keep the OE geometry. So they came to the conclusion that for a 17x9" wheel, the 27 offset was the best option based on where the centerline of the wheel was located when compared to the factory wheels/suspension.

What makes moving the center of the wheel further outwards better in this case as opposed to keeping it as far inboard as possible and still able to fit the 255-275 tires? -Or is that +27 offset the highest offset value you can use and still physically fit those 275s?

Everything I've ever seen or heard about scrub radius says that when you move the wheel centerline further outward, -and in this case increase the already positive scrub-, you cause a heavier steering wheel effort, a more numb steering feedback for the driver and can cause braking instability under heavy decel.

In most cases, the benefits of reducing offset - such as wider track, so less lateral weight transfer - outweigh the costs - such as increased scrub. So we do it. Note, also, that the downsides of increased scrub really only apply to tight turns, but, again, an autocrosser who is allowed to alter offset wants the wider track so much he or she ignores the added scrub.